This invention relates generally to transmissive screens, and more particularly to transmissive screens suitable for use in rear projection systems.
Rear projection screens are generally designed to transmit an image projected onto the rear of the screen into a viewing space. The viewing space of the projection system may be relatively large (e.g., rear projection televisions), or relatively small (e.g., rear projection data monitors). The performance of a rear projection screen can be described in terms of various characteristics of the screen. Typical screen characteristics used to describe a screen""s performance include gain, viewing angle, resolution, contrast, the presence of undesirable artifacts such as color and speckle, and the like. It is generally desirable to have a rear projection screen that has high resolution, high contrast and a large gain. It is also desirable that the screen spread the light over a large viewing space. Unfortunately, as described more fully below, as one screen characteristic is improved, one or more other screen characteristics often degrade. For example, in order to increase the screen gain using the same overall structure, one must decrease the viewing angle over which the screen is readily observable. As a result, certain tradeoffs are made in screen characteristics and performance in order to produce a screen that has overall acceptable performance for the particular rear projection display application.
Rear projection display applications requiring high resolution are becoming more prominent. For example, high definition and digital television formats have relatively high resolution requirements. As a result, screens used in such rear projection televisions must be able to resolve the higher resolution images. Rear projection desktop data monitors also have high resolution requirements. As typical rear projection screens are modified to accommodate the higher resolution, the above-described tradeoffs tend to degrade other performance characteristics of the screen. Thus, there remains a need for screens that have improved the overall performance while meeting the minimum performance criteria necessary for the rear projection display application in which the screen is used.
Generally, the present invention relates to rear projection screen assemblies. In one particular embodiment, the screen assembly includes a diffuser and a polarizer. The diffuser has particles dispersed within a binder. The particles and the binder have different refractive indices and are chosen such that the diffuser spreads light of an initial polarization state into a cone of angles while substantially preserving the initial polarization state of the light. The diffuser also substantially depolarizes a portion of the light that is laterally scattered in the diffuser. The polarizer is disposed to receive light transmitted through the diffuser. The polarizer transmits light of the initial polarization state while absorbing a portion of the laterally scattered light that has a different polarization state.
In another embodiment of the invention a diffuser and polarizer are combined in a screen assembly in a manner such that the modulation depth of diffused light exiting the diffuser is less than the modulation depth of the diffused light after it is transmitted through the polarizer. In such an embodiment, the resolution of the overall screen assembly may be made higher while also improving other characteristics of the screen assembly.
In still another embodiment, a screen assembly including a diffuser and a polarizer can be made which has overall high-performance screen characteristics. Such a screen may have a gain of at least about 0.5, exhibit slight or no scintillation, have a viewing angle of at least 50 degrees and a modulation depth of at least about 0.80.
In still another embodiment, the screen assembly includes a diffuser having particles dispersed in a binder, a polarizer laminated to the diffuser, a glass sheet laminated to the polarizer on the viewer side of the screen assembly and an anti-reflective coating the outside of the glass sheet. The screen assembly may have a viewing angle of at least about 65 degrees, a peak gain of at least about 0.65 and a modulation depth of at least about 0.90. The screen assembly may further have, at most, only slight observable scintillation.
An improved projection system is provided in still another embodiment of the invention. The projection system includes a projector used to generate a polarized image made up of pixels. A bulk diffusing element is disposed to receive and diffuse the projected image to a prescribed viewing angle. The diffusing element maintains the polarization state of light corresponding to a projected pixel of the image as the light is transmitted through the diffuser at a location corresponding to the projected pixel to a greater extent than it maintains the polarization state of light corresponding to the projected pixel that is laterally scattered by the diffuser such that it exits the diffuser at a location corresponding to a different pixel. The system also includes an absorbing polarizer following the diffuser in the light path. The polarizer is oriented so as to pass the light corresponding to the projected pixel that transmits through the diffuser at the location corresponding to the projected pixel.
The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and detailed description that follow more particularly exemplify these embodiments.